Anticorrosion treatment

ABSTRACT

An anticorrosion treatment of an aluminium/zinc alloy surface is disclosed. The treatment includes the steps of forming on the alloy surface a coating of a solution which contain 5-40 grams of molybdenum per litre of the solution, 2-19% by volume of a phosphoric acid, and a surface etchant. The treatment also includes drying the coating to form a dried coating having a loading of molybdenum of at least 10 mg/m 2  of the dried coating and of phosphorus of at least 15 mg/m 2  of the dried coating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an anti-corrosion treatment ofaluminium/zinc alloy surfaces.

In particular, although by no means exclusively, the present inventionrelates to an anticorrosion treatment of steel strip having a coating ofan Al/Zn alloy.

2. Description of Related Art

Zinc, aluminium and/or combinations of aluminium and zinc are widelyused as surface coatings, particularly but not exclusively for steel forprotection against corrosion. In practice, however, the zinc or Al/Zncoatings are susceptible to white corrosion (white rust) or blackcorrosion (black rust) respectively when exposed to the atmosphere dueto reactions with moisture. Such corrosion is detrimental to the surfaceappearance and generally makes coated steel substrates unacceptablecommercially despite the fact that the overall service life of thecoated steel may remain the same. Further the formation of corrosionproducts generally interferes with finishing operations. The ability toresist such corrosion is referred to herein as wet stack performance.

In order to inhibit the formation of corrosion on coated surfaces it isgenerally accepted that the treatment of a surface with a chromateimparts anticorrosive properties and this type of treatment is generallyreferred to as chromate passivation. However, chromate is highly toxicto exposed workers and, due to its high toxicity, disposal of chromiumresidues is difficult. Further, in various markets yellow discolourationof treated coated surfaces caused by the chromate is considered to be anunacceptable product attribute.

In order to overcome the problems associated with chromate passivation,phosphate coatings have been used. However the anticorrosion propertiesof phosphate have been found to be far inferior to the above-mentionedchromate treatment.

U.S. Pat. No. 4,385,940 assigned to Kobe Steel, Limited discloses ananticorrosive treatment for preventing white rust on galvanized steelwhich includes the steps of applying to the surface of a galvanizedsteel sheet an acidic solution containing molybdic acid or a molybdatein a concentration of 10-200 g/l (calculated as molybdenum) and adjustedto a pH of 1 to 6 by addition of an organic or inorganic acid. However,whilst the anticorrosive treatment described in the US patent works wellfor galvanised steel, it has been found that the corrosion resistance ofAl/Zn alloy surfaces treated with the above solution is inferior to thechromate treated substrates under certain conditions and the treatedsurfaces suffer from an undesirable degree of discolouration. Moreover,molybdate treated surfaces of such material have been observed to changefrom a pale yellow/blue to a strong green colour when stored for periodsof time in excess of 24 hours.

International application PCT/US97/00012 (WO97/27001) in the name ofHenkel Corporation discloses an anti-corrosive treatment foraluminium/zinc alloy surfaces which is based on the use of a solutionthat includes phosphate anions and molybdenum anions and/or compounds inwhich the molybdenum has an oxidation state less than +6.

It is an object of the present invention to provide an alternativeanticorrosion treatment for Al/Zn alloy surfaces.

SUMMARY OF THE INVENTION

According to the present invention there is provided an anticorrosiontreatment of an aluminium/zinc alloy surface which includes the stepsof:

(i) forming on the surface a coating of a solution which contains 5-40grams of molybdenum per litre of the solution, 2-19% by volume of aphosphoric acid, and a surface etchant; and

(ii) drying the coating to form a dried coating having a loading ofmolybdenum of at least 10 mg/m² of the dried coating and of phosphorusof at least 15 mg/m² of the dried coating.

The amount of 2-19 volume % of the phosphoric acid equates to 9.1 to86.9 grams phosphorus per litre of the solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The applicant has found in laboratory and outdoors testing that coatingshaving the above molybdenum and phosphorous loadings exhibit excellentlevels of corrosion resistance and lower levels of discolouration thanprior art coatings.

Whilst not wishing to be bound by the following comments in thisparagraph, the applicant believes that the excellent performance ofcoatings having the above loadings of molybdenum and phosphorus is dueto the above-described anticorrosion treatment initially forming a layerof zinc phosphate on the surface and then forming a layer of molybdenumphosphate on that layer.

The applicant has found that the excellent performance of coatingshaving the above loadings of molybdenum and phosphorus was achievedwithout the need to ensure that the molybdenum in the solution had anoxidation state less than +6 and without the use of reducing agents inthe solution to achieve this outcome, as is the case with Internationalapplication PCT/US97/00012 (WO97/27001).

In any given situation, the amounts of molybdenum and phosphoric acidthat are necessary to obtain the product loadings of at least 10 mg/m2molybdenum and at least 15 mg/m² phosphorus depends on a range offactors, such as, by way of example, the zinc concentration of the Al/Znalloy, pH of the solution, and the thickness of the final coating.

Preferably the Al/Zn alloy contains 25-75 wt.% aluminium.

More preferably the Al/Zn alloy is aluminium rich.

Preferably the surface coating formed in step (i) is 3-5 micron thick.

The surface coating may be formed by any suitable means.

By way of example, the coating may be formed by applying the solution tothe Al/Zn alloy surface by means of a roller-coater.

Alternatively, the coating may be formed by firstly dipping the Al/Znalloy surface into a bath of the solution or spraying the solution ontothe surface to form a coating having a thickness greater than 5 micronand thereafter removing excess solution by means of a squeegee roller orother suitable means.

The solution may be applied to the Al/Zn alloy surface at any suitabletemperature.

Preferably the solution application temperature is less than 35° C.

Preferably the pH of the solution is less than 3 when it is initiallyapplied to the Al/Zn alloy surface.

More preferably the pH of the solution is less than 2.6 when it isinitially applied to the Al/Zn alloy surface.

Preferably the molybdenum in the solution has an oxidation state of +6.

The coating formed in step (i) may be dried by any suitable means instep (ii) that ensures the coating is thoroughly dried.

Preferably step (ii) includes drying the coating formed in step (i) attemperatures of at least 60° C.

More preferably step (ii) includes drying the coating formed in step (i)by induction heating.

Preferably, the dried coating formed in step (ii) is 20-100 nanometersthick.

More preferably the dried coating is 30-50 nanometers thick.

Preferably the molybdenum loading of the dried coating is at least 15gm/m² of the coating.

Preferably the phosphorus loading of the coating is at least 20 gm/m² ofthe coating.

Preferably the Al/Zn alloy is a coating on a steel strip.

Preferably the coating is continuous.

Preferably the solution contains 5-30 g/l molybdenum.

Preferably the solution contains at least 13.5 g/l molybdenum.

Preferably the solution contains less than 20 g/l molybdenum.

Preferably the molybdenum is added as a salt.

Preferably the molybdenum salt is ammonium molybdate.

Other suitable molybdenum salts include sodium and potassium molybdate.

Preferably the phosphoric acid is concentrated phosphoric acid. The term“concentrated” is understood to mean that the acid is in the form of anaqueous solution in which at least 80 volume % of the solution is acidand less than 20 volume % is water.

Preferably the solution contains 2-10% by volume of the phosphoric acid.

More preferably the solution contains at least 3 volume % of thephosphoric acid.

It is preferred particularly that the solution contain at least 4 volume% of the phosphoric acid.

Preferably the phosphoric acid is orthophosphoric acid.

Preferably the surface etchant is a fluorine containing compounds, suchas sodium fluoride.

Preferably the solution contains at least 0.3 g/l fluorine.

More preferably the solution contains at least 0.5 g/l fluorine.

The solution may contain other constituents.

By way of example, the solution may contain up to 5 g/l vanadium.

According to the present invention there is also provided an Al/Zn alloysurface treated in accordance with the above-described anticorrosiontreatment.

According to the present invention there is also provided a solution foruse in the above-described anticorrosion treatment which includes 5-40g/l molybdenum, 2-19% by volume of a phosphoric acid, and an etchant.

Preferably the molybdenum in the solution has an oxidation state of +6.

Preferably the solution contains 5-30 g/l molybdenum.

More preferably the solution contains more than 13.5 g/l molybdenum.

More preferably the solution contains less than 20 g/l molybdenum.

In order to investigate the performance of the present invention theapplicant carried out a series of wet-stack laboratory experiments onZINCALUME panels treated with the range of treatment solutionssummarised in Table 1.

In order to simulate conditions expected in a commercial production linethe treatment solutions were applied by dipping the panels for 4 secondsin the treatment solution and then sheen spinning excess solution. Thecoatings on the panels were then thoroughly dried using a convection airdrier.

The molybdenum, vanadium, phosphorus and fluorine in the treatmentsolutions used in samples 2-8 were added as ammonium molybdate, ammoniumvanadate, orthophosphoric acid, and sodium fluoride respectively. Theorthophosphoric acid was either 81 or 85% aqueous orthophosphoric acid.

The pH of the treatment solutions varied between 1.5 and 2.2.

The dried coatings of samples 2-8 were 20-100 nanometers thick. Thedried coatings of samples 2-5 had loadings of molybdenum and phosphorusabove 10 and 40 mg/m², respectively.

The wet-stack experiments were carried out over a 4 week period and at40° C.

Details of the colour and corrosion resistance of each panel and theconclusion of the wet-Stack experiments are set out in Table 1.

TABLE 1 Sample Colour After 24 Hours After Pass or Fail No TreatmentSolution Passivation Corrosion Resistance Corrosion Test 1 ChromatedZINCALUME Clear No sign of corrosion Pass 2 27 g/L Mo, 0.5 g/L V, 10%H₃PO₄ (45.6 g/L P), Blue & green. Colour varied No sign of blackstaining. 5% of red Pass 0.4 g/L F, pH 1.5 at edges considerably rust oncut edges 3 13.5 g/L Mo, 2 g/L V, 5% H₃PO₄ (45.6 g/L P), Light Tan Nosign of black staining. 5% of red Pass 0.7 g/L F, pH 1.5 rust on cutedges 4 13.5 g/L Mo, 0 g/L V, 5% H₃PO₄ (22.8 g/L P), Very Light Tan 5%black staining (very light grey in Pass 0.7 g/L F colour). 5-10% redrust around the cut edges 5 13.5 g/L Mo, 2 g/L V, 2% H₃PO₄ (9.1 g/L P),Light Tan No sign of black staining. 15-20% Pass 0.7 g/L F, pH 1.9white/grey corrosion 6 13.5 g/L Mo, 2 g/L V, 5% H₃PO₄ (22.8 g/L P),Light Tan 100% black staining and 100% red rust on Fail 0 g/L F cutedges. Note 2/6 panels had no sign of black staining (problem withvariability) 7 8 g/L MO, 1 g/L V, 1.5% H₃PO₄ (22.8 g/L P), Clear 100%black staining and 100% red rust on Fail 0 g/L F cut edges 8 5.4 g/L MO,1 g/L V, 1.5% H₃PO₄ (22.9 g/L P), Clear 100% black staining and 100% redrust on Fail 0.2 g/L F cut edges 9 Untreated ZINCALUME Clear 100% blackstaining. 100% red rust on Fail cut edges

It is clear from the table that the treated panels of samples 3-5 inaccordance with the present invention had comparable colour and samples2-3 had comparable corrosion resistance to the chromate ZINCALUME panelof sample 1 and significantly better corrosion resistance than thetreated panels of panels 6-8 and the untreated ZINCALUME panel of sample9.

Although the invention has been described with reference to specificexamples, it would be appreciated by those skilled in the art that theinvention may be embodied in many other forms.

What is claimed is:
 1. An anticorrosion treatment of an aluminium/zincalloy surface which includes the steps of: (i) forming on the surface acoating of a solution which contain 5-40 grams of molybdenum per litreof the solution, 2-19% by volume of a phosphoric acid, and a surfaceetchant; and (ii) drying the coating to form a dried coating having aloading of molybdenum of at least 10 mg/m² of the dried coating and ofphosphorous of at least 15 mg/m² of the dried coating; wherein themolybdenum in the solution has an oxidation state of +6.
 2. Thetreatment defined in claim 1 wherein the Al/Zn alloy contains 25-75 wt.% aluminium.
 3. The treatment defined in claim 2 wherein the Al/Zn alloyis aluminium rich.
 4. The treatment defined in claim 1 wherein step (i)includes forming the surface coating by applying the solution to theAl/Zn alloy surface at a solution application temperature of less than35° C.
 5. The treatment defined in claim 1 wherein the pH of thesolution is less than 3 when it is initially applied to the Al/Zn alloysurface.
 6. The treatment defined in claim 5 wherein the pH of thesolution is less than 2.6 when it is initially applied to the Al/Znalloy surface.
 7. The treatment defined in claim 1 wherein the solutioncontains 5-30 g/l molybdenum.
 8. The treatment defined in claim 1wherein the solution contains at least 13.5 g/l molybdenum.
 9. Thetreatment defined in claim 1 wherein the solution contains less than 20g/l molybdenum.
 10. The treatment define in claim 1 wherein the solutioncontains 2-10% by volume of the phosphoric acid.
 11. The treatmentdefined in claim 1 wherein the solution contains at least 4% by volumeof the phosphoric acid.
 12. The treatment defined in claim 1 wherein thesurface etchant is a fluorine-containing compound and the solutioncontains at least 0.3 g/l fluorine.
 13. The treatment defined in claim12 wherein the solution contains at least 0.5 g/l fluorine.
 14. An Al/Znalloy having a corrosion resistant surface obtained by: (i) forming onthe surface a coating of a solution which contain 5-40 grams ofmolybdenum per litre of the solution, 2-19% by volume of a phosphoricacid, and a surface etchant, wherein the molybdenum in the solution hasan oxidation state of +6; and (ii) drying the coating to form a driedcoating having a loading of molybdenum of at least 10 mg/m² of the driedcoating and of phosphorous of at least 15 mg/m² of the dried coating.15. A solution for use in an anticorrosion treatment for a surface of analuminum/zinc alloy, said solution comprising 5-40 g/l molybdenum, 2-19%by volume of phosphoric acid, and a surface etchant, wherein themolybdenum in the solution has an oxidation state of +6; and whereinsaid solution is adapted to form a dried coating on said surface of saidaluminum/zinc alloy, said dried coating having a loading of molybdenumof at least 10 mg/m² of the dried coating and a loading of phosphorousof at least 15 mg/m² of the dried coating.